Compositions which produce a color change upon exposure to energy in the form of light or heat are of great interest in producing images on a variety of substrates. As an example, optical disks represent a significant percentage of the market for data storage of software as well as of photographic, video, and/or audio data. Typically, optical disks have data patterns embedded thereon that can be read from and/or written to one side of the optical disk, and a graphic display or label printed on the other side of the optical disk.
In order to identify the contents of the optical disk, printed patterns or graphic display information can be provided on the non-data, or label, side of the optical disk. The patterns or graphic display can be both decorative and provide pertinent information about the data content of the optical disk. In the past, commercial labeling has been routinely accomplished using screen-printing methods. While this method can provide a wide variety of label content, it tends to be cost ineffective for production of less than about 400 customized disks because of the fixed costs associated with preparing a stencil or combination of stencils and printing the desired pattern or graphic display.
In recent years, the significant increase in the use of optical disks for data storage by consumers has increased the demand to provide customized labels to reflect the content of the optical disk. Most consumer-available methods of labeling are limited to either handwritten descriptions which lack professional appearance, quality and variety; or preprinted labels which may be affixed to the optical disk, but which can also adversely affect the disk performance upon spinning at high speeds.
A number of materials which can produce a color change upon exposure to energy are known. For example, such color forming materials are used in thermal printing papers, instant imaging films, and the like. These materials typically use a multi-layered composite structure and often additional processing steps. Most often, these technologies can require relatively high heat flux over somewhat long periods of time. For example, some of these methods utilize carbon dioxide lasers having energy densities of greater than 3.0 J/cm2 for exposure times of greater than about 100 μsec. Therefore, there are limitations as to the types of materials which can be used as a substrate and the marking speeds.
A recent development in color forming compositions, including the use of leuco dyes, has provided color forming compositions that can be developed using energy sources to form an image. Though leuco dyes have been used with energy sources such as thermal and light-directed imaging, they suffer from the limitation of image stability because the color forming reaction is often reversible and they display poor to moderate light fastness properties. For these and other reasons, the need still exists for color forming compositions with increased image permanence.